The present invention relates generally to radio navigation systems and methods for use with aircraft and the like, and more particularly to a microwave phase reference system and method which may be used for omni-directional, bi-directional or uni-directional navigation.
The very high frequency omni-directional range (VOR) navigation system is a well known phase reference omni-directional navigation system which is used extensively throughout the world to provide aircraft with flight path bearing information. In this system, two signals are radiated by a VOR ground station, and the station produces a spatially rotating electromagnetic field. One of the signals comprises a reference phase signal which is radiated omni-directionally, and the other signal comprises a variable phase signal which has a phase that varies linearly with azimuth angle relative to the reference signal. The reference phase signal is transmitted as a 30 Hz frequency modulated 9960 Hz subcarrier on a radio frequency (RF) carrier in the VHF frequency range of 112-118 MHz. The variable phase signal comprises a 30 Hz amplitude modulation which is impressed upon the RF carrier by utilizing an antenna system which produces a non-symmetrical field, such as a cardioid or a lamicon pattern, and feeding the antenna system so as to cause the field to rotate at eighteen hundred RPM or, expressed otherwise, at 30 Hz. A cooperating aircraft receiver provides bearing information relative to the VOR station by measuring the phase difference between the reference and the variable phase signals.
Conventional VOR (CVOR) systems are susceptible to a number of errors. The most common error is multipath error caused by reflections from surface irregularities or obstacles above or below the ground plane of the VOR site. Such surface reflections may be due, for example, to varying topography or to the presence of buildings, fences, trees, etc . . . Multipath errors result from the vector addition of the direct and reflected field components of the received signal. The amount of error is dependent upon the relative magnitude of the direct and reflected signals, the phase difference between the signals, and the difference in azimuth between the receiver and reflector. For example, the course error, .alpha., may be calculated by use of the formula ##EQU1## where K is the amplitude ratio of direct to reflected signals and .phi. is the phase difference in azimuth between the direct and reflected signals. Accordingly, if K=0.3 and .phi.=30.degree., course error .alpha.=7.5 degrees.
Multipath errors may be minimized to some extend by use of a Doppler VOR (DVOR) system in which the reference phase signal is radiated by an omni-directional antenna as a 30 Hz amplitude modulated RF carrier signal, and the azimuth dependent variable phase signal is generated in space as a 9960 Hz carrier sideband which is frequency modulated at 30 Hz by the doppler effect of sequentially switched antennas disposed in a ring about the omni-directional reference phase signal antenna. Typically, a DVOR array comprises fifth antennas disposed around a horizontal aperture having a diameter of about five wavelengths (13.5 meters). The physical size of a CVOR or DVOR antenna array, associated counterpoise and required large ground plane area precludes operation from sites having limited space such as offshore oil drilling platforms, heliports and water vessels. Control of VOR radial alignment is primarily limited to adjustment of one course to coincide with a selected azimuth (Magnetic North) bearing. Course errors inherent to VOR systems include octantal error, quadrantal error, duantal error in addition to other errors caused by counterpoise effect. Mountain top VOR stations often encounter a limited ground plane (less than 20 wavelengths) thereby radiating negative angle signals which can produce multipath signal conditions. In addition, since known phase reference navigational systems employ spatially rotating fields, i.e., antenna patterns, it is necessary for the receiver to receive signals radiated by all elements of the antenna array. Thus, uni-directional or bi-directional operation is not possible.
It is desirable to provide a new and improved navigation system which avoids the foregoing and other disadvantages of known systems, and it is to this end that the present invention is directed.